Modulating the Steric, Electronic, and Catalytic Properties of Cp* Ruthenium Half-Sandwich Complexes with β-Diketiminato Ligands

Five different types of β-diketiminate ligands,
bearing electron-donating
to strongly electron-withdrawing substituents, were synthesized and
used in the synthesis of Cp* ruthenium complexes (Cp* = η5-C5Me5). One series consists
of complexes with a covalent RuIII–Cl bond,
and the other series features a reduced RuII center, where
the chloride is abstracted by treatment of the corresponding RuIII compounds with Zn or Mg. All compounds were characterized
by single-crystal X-ray diffraction, UV–visible spectroscopy,
and cyclic voltammetry. In the case of RuII complexes,
solution NMR techniques provided key information regarding the electronic
and structural differences induced by the different β-diketiminate
ligands employed. Capitalizing on the facile reduction–oxidation
cycle of the Cp* ruthenium β-diketiminato complexes, catalytic
atom transfer radical addition (ATRA) and cyclization (ATRC) reactions
were performed on relevant substrates. The turnover rates are strongly
dependent on the type of β-diketiminate used, where ligands
with electron-withdrawing substituents, i.e., trifluoromethyl groups,
provided complexes that efficiently catalyze the addition of CCl4 or toluenesulfonyl chloride to styrene. In contrast, complexes
with electron-donating substituents on the β-diketiminate promoted efficient ATR cyclization of N-allyl-N-phenyltrichloroacetamide and 2,2,2-trichloroethyl ether. Thus, the
overall product conversion and yield are dependent on matching the
ligand substitution pattern of the catalyst to the type of substrate.